The above-listed applications are commonly assigned with the present invention and are incorporated herein by reference as if reproduced herein in their entirety.
The present invention is directed, in general, to logic gates and, more specifically, to a low power differential logic gate and methods of operation and manufacturing thereof.
Digital systems are used extensively in computation and data processing, controls, communications and measurement. Digital systems use digital signals that may only assume discrete values. Typically, digital systems use binary signals that employ only two values. Since such systems only use two distinct values, errors caused by component variations are minimized. As a result, a digital system may be designed such that, for a given input, an output thereof is exactly correct and repeatable. This gives rise to the extreme accuracy for which digital systems are well known.
Analog systems, on the other hand, use analog signals that vary continuously over a specified range. Analog systems are thus particularly vulnerable to error, depending on the accuracy of the components used therein. Since digital systems are generally capable of greater accuracy and reliability than analog systems, many tasks formerly performed by analog systems are now performed exclusively by digital systems.
One basic building block of digital systems is a logic gate. Conventional logic gates have one output and one or more inputs. The number of inputs is called the xe2x80x9cfan-inxe2x80x9d of the gate. The state of the output is completely determined by the state(s) of the input(s). Conventional logic gates are typically created by coupling a number of transistors together to perform a Boolean function (e.g., AND, OR, NOT). The logic gates are then coupled together to form a multi-layer circuit that is capable of performing logical functions (e.g., arithmetic functions).
Increasing processing power is a continuing goal in the development of processors such as microprocessors or digital signal processors (DSPs). Processor designers are generally familiar with three ways to increase the processing power of a central processing unit (CPU). The CPU""s clock frequency may be increased so that the CPU can perform a greater number of operations in a given time period. Processors are designed to operate at increasingly high clock frequencies. While a higher clock frequency generally results in increased processing power, the higher clock frequency also increases power dissipation, resulting in higher device operating temperatures. Processor designers, therefore, must address these additional problems to avoid catastrophic device failures.
Another way to increase processing power is to increase input and output data bus width, thereby allowing the CPU to process a greater amount of code and data. Early processors were packaged using dual in-line packaging (DIP) technology. Increasing the width of the data buses was both expensive and unrealistic, often resulting in extremely large device packages. Today, with the use of pin grid array (PGA) packaging, increasing the size of the data buses no longer poses a packaging problem. Of course, a larger number of transistors is required to process the additional information conveyed by the wider data buses, which translates into increased power dissipation by the processor.
Yet another way to increase processing power is to change the internal architecture of the processor to overlap the execution of instructions by, for example, superscaling. This method also requires the addition of a large number of transistors, since entire processing stages or execution units must be duplicated. Performing a large number of instructions in parallel may also result in data dependency problems. Further, the additional transistors also increase power dissipation.
With the rise of portable and personal computing, power dissipation has become an important factor in the design of processors. Processors employed in desktop or server-based applications may be designed to maximize speed with little regard for power consumption. Processors employed in laptop, palmtop or other portable devices, however, must be designed with power consumption in mind due to the often limited capabilities of the batteries powering the portable devices. Since the processor includes a large number of logic gates, reducing power dissipation in the individual logic gates can accordingly reduce the power consumption of the processor as a whole.
Accordingly, what is needed in the art is a logic gate operable at high frequency that overcomes the disadvantages of prior art logic gates.
To address the above-discussed deficiencies of the prior art, the present invention provides a logic gate and methods of operation and manufacturing thereof. In one embodiment, the logic gate comprises complementary first and second computational blocks having first and second sets of binary inputs, respectively. The first computational block develops an output binary digit that is a function of a weighted sum of a first set of input binary digits presented at the first set of binary inputs. The second computational block develops a complementary output binary digit that is a function of a weighted sum of a second set of input binary digits presented at the second set of binary inputs. The logic gate further comprises a cross-coupled differential load. The cross-coupled differential load includes a first load circuit coupled to the first computational block and driven by the complementary output binary digit. The cross-coupled differential load further includes a second load circuit coupled to the second computational block and driven by the output binary digit.
The present invention therefore introduces the broad concept of employing complementary computational blocks to compute complementary output binary digits. The complementary output binary digits are employed to control the load circuits to accelerate a pace at which the complementary output binary digits transition from state to state.
In one embodiment of the present invention, the cross-coupled differential load cooperates with the first and second computational blocks to increase a noise tolerance of the logic gate. The cross-coupled differential load reduces the need for additional noise suppression circuitry (see, e.g., the ""367 application), thereby reducing a number of transistors required to implement the logic gate.
In one embodiment of the present invention, the logic gate computes the output binary digit and the complementary output binary digit without employing a timing signal. While some logic gates of the prior art required a timing signal to synchronize the operation of the logic gate, the logic gate of the present invention is capable of operating asynchronously.
In one embodiment of the present invention, the first load circuit cooperates with the first computational block to implement a first threshold function. The output binary digit may thus be further a function of the first threshold function. In a related embodiment, the second load circuit cooperates with the second computational block to implement a second threshold function. The complementary output binary digit may further be a function of the second threshold function.
In one embodiment of the present invention, the first load circuit comprises a first transistor and the second load circuit comprises a second transistor. The first and second transistors may be sized proportional to xcex8 and xcex8xe2x88x921, respectively.
In one embodiment of the present invention, the first and second computational blocks employ identical topologies, thus simplifying design of the overall logic gate.
In one embodiment of the present invention, ones of the second set of input binary digits are inverses of corresponding ones of the first set of input binary digits. In an alternative embodiment, ones of the second set of input binary digits are identical to corresponding ones of the first set of input binary digits. Whether the ones of the second set of input binary digits are inverses or identical to the ones of the first set of input binary digits depend on the polarities of the transistors employed in the first and second computational blocks.
In one embodiment of the present invention, the first and second computational blocks comprise n-channel metal oxide semiconductor (NMOS) transistors, each of the first and second load circuits comprising a p-channel metal oxide semiconductor (PMOS) transistor. In an alternative embodiment, the first and second computational blocks comprise PMOS transistors, each of the first and second load circuits comprising an NMOS transistor. In either case, the drive signals to the first and second load circuits may be taken directly from the second and first computational blocks. In another alternative embodiment, the first and second computational blocks comprise one polarity of MOS transistors (whether NMOS or PMOS), each of the first and second load circuits comprising an identical polarity of MOS transistor and an inverter. The inverters are used to invert the output binary digit and the complementary output binary digit before application to the first and second load circuits.
In one embodiment of the present invention, both the first and second computational blocks comprise NMOS transistors, ones of the second set of input binary digits being inverses of corresponding ones of the first set of input binary digits. In another embodiment, the first computational block comprises NMOS transistors while the second computational block comprises PMOS transistors. Ones of the second set of input binary digits are therefore identical to corresponding ones of the first set of input binary digits.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.